In recent years, the digital coherent technology and the multilevel modulation technology, which combine digital signal processing and coherent detection, have received attention to further increase the speed and capacity of an optical transmission system. Research and development of an optical transmission system that attains 100 Gb/s per wavelength using polarization multiplexing and a QPSK (Quadrature Phase Shift Keying) modulation format and devices for the 100-Gb/s optical transmission system has extensively been pursued now. In addition, to make the 100-Gb/s optical transmission system more sophisticated and improve its transmission quality, applying digital signal processing such as Nyquist filter or pre-equalization to the transmission end has been examined. Furthermore, utilization of a higher-order multilevel modulation format such as QAM (Quadrature Amplitude Modulation) has also been examined to realize a transmission technology of 400-Gb/s class per wavelength.
FIG. 40 shows an example of the arrangement of a general optical transmitter for 100 Gb/s transmission. FIG. 40 shows a transmission block for one polarized wave out of polarization multiplexing. The optical transmitter shown in FIG. 40 includes a DSP (Digital Signal Processor) unit 100 that performs digital signal processing of transmission data Data, multiplexers (MUXs) 101-I and 101-Q that multiplex symbols output from the DSP unit 100, optical modulator driver circuits 102-I and 102-Q that amplify signals output from the MUXs 101-I and 101-Q, a laser diode (LD) 103, and an optical I/Q modulator 104 that modulates continuous light from the LD 103 by the output signals from the optical modulator driver circuits 102-I and 102-Q and outputs the signal. FIG. 41A is a view showing the output signal of the MUX 101-I. FIG. 41B is a view showing the output signal of the optical modulator driver circuit 102-I. FIG. 41C is a constellation diagram showing, on a plane, the optical output signal of the optical I/Q modulator 104.
The DSP unit 100 includes an FEC (Forward Error Correction) encoding unit 1000 that performs FEC encoding for the transmission data Data, and a symbol mapping unit 1001 that executes symbol mapping according to a modulation format for the signal that has undergone the FEC encoding. As described above, since the 100-Gb/s optical transmission system uses the modulation format of QPSK, the electrical signal that drives the optical I/Q modulator 104 is a binary signal. For this reason, the optical modulator driver circuits 102-I and 102-Q need to perform a limit operation (operation of limit-amplifying both a small signal and a large signal up to a desired amplitude value) to improve the eye opening of the modulator driving waveform. In other words, requirement of linearity (characteristic for linearly amplifying an input signal) is not high in the optical modulator driver circuits 102-I and 102-Q used in the 100-Gb/s optical transmission system.
FIG. 42 shows an example of the arrangement of an optical transmitter capable of using transmission end signal processing and also coping with a higher-order modulation format such as QAM. FIG. 42 also shows a transmission block for one polarized wave out of polarization multiplexing. The optical transmitter shown in FIG. 42 includes a DSP unit 200 that performs digital signal processing of the transmission data Data, MUXs 201-I and 201-Q that multiplex symbols output from the DSP unit 200, D/A converters (DACs: Digital to Analog Converters) 202-I and 202-Q that convert data output from the MUXs 201-I and 201-Q into analog signals, optical modulator driver circuits 203-I and 203-Q that amplify signals output from the DACs 202-I and 202-Q, an LD 204, and an optical I/Q modulator 205 that modulates continuous light from the LD 204 by the output signals from the optical modulator driver circuits 203-I and 203-Q and outputs the signal. FIG. 43A is a view showing the output signal of the DAC 202-I. FIG. 43B is a view showing the output signal of the optical modulator driver circuit 203-I. FIG. 43C is a constellation diagram showing, on a plane, the optical output signal of the optical I/Q modulator 205.
The DSP unit 200 includes a pre-equalization unit 2002 that performs, for the signal, pre-equalization processing of wavelength dispersion or nonlinear response of an optical modulator, a signal spectrum shaping unit 2003 that performs spectrum shaping (Nyquist filter) processing for the signal to suppress inter-channel crosstalk at the time of WDM (Wavelength Division Multiplexing) transmission, and a transmission FE equalization unit 2004 that performs, for the signal, transmission FE (Forward Equalizer) equalization for the optical modulator, in addition to an FEC encoding unit 2000 that performs FEC encoding for the transmission data Data and a symbol mapping unit 2001 that executes symbol mapping according to a modulation format for the signal that has undergone the FEC encoding. The functions of the pre-equalization unit 2002, the signal spectrum shaping unit 2003, and the transmission FE equalization unit 2004 can be ON/OFF-controlled as needed (see literature “3rd, New Optical Transmission Technologies by Digital Signal Processing—100 G and Beyond—, Proceedings of the IEICE, Optical Communication System Technical Committee, pp. 9-13, 2012).
The arrangement shown in FIG. 42 is largely different from the conventional 100-Gb/s optical transmitter shown in FIG. 40 in that linearity is important. When the simple QPSK format is used, the electrical signal that drives the optical I/Q modulator 104 is a binary signal “0” or “1”, as described above. However, when Nyquist filter or pre-equalization processing is applied, or a higher-order multilevel modulation format with amplitude modulation such as QAM is used, the electrical signal that drives the optical I/Q modulator 205 is not a simple “0” or “1” signal but a signal finely including information in the amplitude axis direction. As an example easy to understand, when a 16-QAM format is used, the electrical signal that drives the optical I/Q modulator 205 is a quaternary signal, as shown in FIG. 43C.
As described above, when transmission end signal processing or QAM format is used, the electrical signal that drives the optical modulator finely includes information in the amplitude axis direction. For this reason, the optical modulator driver circuit needs to linearly respond, that is, linearly amplify the input signal. Additionally, an optical transmitter as shown in FIG. 42 can cover a conventional system that handles a binary signal.